In one type of prior accelerometer, a pendulum is suspended from a support by a flexure hinge that constrains movement of the pendulum except movement along the accelerometer's sensitive axis. A force sensing element is connected along the sensitive axis between the pendulum and the support, the force sensing element being attached to the pendulum at the end of the pendulum opposite the flexure hinge. An acceleration along the sensitive axis results in a compression or tension force on the force sensing element. This force is converted into an electrical signal that indicates both the direction and magnitude of the acceleration.
One well known type of force sensing element comprises one or more quartz beams that are forced to vibrate in a particular normal mode by means of electrodes on the beam surfaces and an oscillator circuit connected to the electrodes. The normal mode frequencies of the beams change in response to changes in applied compression or tension forces along the longitudinal beam axes. For a single beam force sensing element, one suitable vibration mode is a flexural mode in which the beam vibrates from side-to-side in a direction normal to the accelerometer's sensitive axis and normal to the lengthwise pendulum axis, i.e., the pendulum axis extending between the end connected to the flexure hinge and the end connected to the force sensing element. For a dual beam force sensing element, a preferred mode of vibration is a flexural mode in which the beams vibrate from side-to-side in the direction described for the single beam element, but in which the beams are 180.degree. out of phase with one another.
One of the advantages of using a dual beam force sensing element in an out-of-phase vibration mode is that the reaction forces of the beams cancel, and as a result no net reaction force is coupled to the pendulum. For any given dual beam configuration, however, there are a number of other normal modes that can be excited. For example, the beam can undergo side-to-side flexural vibration in-phase, rather than out of phase. The desired out-of-phase vibration mode may in general be selected by appropriate electrode placement. However for a typical dual beam accelerometer, there is an in-phase normal mode at a frequency that is sufficiently close to the center out-of-phase normal mode frequency to be within the operating range of the instrument. Factors that tend to enhance the in-phase vibration mode are therefore capable of introducing error into the accelerometer output.
It is not uncommon to find that the output of a vibrating beam accelerometer behaves in a highly nonlinear and unpredictable manner at certain frequencies or over certain narrow frequency ranges. The phenomenon of such nonlinear behavior at a certain frequency is referred to as an activity dip. In the past, the problem of activity dips has typically been avoided by adjusting the mass of the pendulum and other parameters in an effort to avoid overlap between activity dips and the operating range of the accelerometer. However, adjustment of such parameters frequently conflicts with other design criteria. For example, the pendulum mass must also be adjusted based upon the desired scale factor and acceleration range, and to avoid spurious resonances due to vibrational inputs. There has therefore been a long-felt need for a technique of eliminating activity dips that would not conflict with other design criteria.